An aza-Diels–Alder approach to nitrogen-containing tetrabenzoacene derivatives

Acenes and N-heteroacenes have been synthesized and studied for over a century because of their fundamentally interesting materials properties and promise for device applications. Within this context, our laboratory has previously synthesized nitrogen-containing tetrabenzo[de,hi,op,st]pentacenes via an aza-Diels–Alder reaction-based approach, and herein, we expand our methodology to obtain substituted, expanded, functionalized, and dimeric tetrabenzoacenes. Overall, our study adds to the limited number of tetrabenzoacene derivatives reported to date and may open further opportunities for these materials in organic optoelectronics applications.

Herein, we report a general and modular aza-Diels-Alder reaction-based approach to substituted, expanded, functionalized, and dimeric nitrogen-containing tetrabenzoacenes (Fig. 1B).First, we synthesize a nitrogen-containing tetrabenzopentacene bearing pendant phenyl rings substituted with dodecyl chains, thus demonstrating incorporation of solubilizing functionalities (Fig. 1B, compound I).Second, we synthesize a nitrogen-containing tetrabenzoheptacene, thus describing expansion of the molecular aromatic core of our compound (Fig. 1B, compound II).Third, we synthesize a nitrogen-containing tetrabenzopentacene featuring amino and bromo functional groups, thus showing installation of reactive electron-withdrawing and electron-donating handles (Fig. 1B, compound III).Fourth, we synthesize a dimeric nitrogen-containing tetrabenzopentacene, thus furnishing a potentially valuable larger model compound (Fig. 1B, compound IV).Last, we comparatively investigate the electronic properties of our various tetrabenzoacene derivatives with ultraviolet-visible (UV-Vis) spectroscopy.Our synthetic strategy affords multiple new nitrogen-containing tetrabenzo[de,hi,op,st]pentacene variants and may enhance the utility of this class of molecules in organic optoelectronics applications.

Results
We began our efforts by synthesizing a nitrogen-containing tetrabenzopentacene derivative bearing pendant phenyl rings substituted with dodecyl chains (Scheme 1).5][36][37][38][39][40][41] Next, we cyclodehydrohalogenated diquinolineanthracene variant 3 under standard conditions (i.e., quinoline as the solvent and KOH as the base), forming two intramolecular carbon-carbon bonds and furnishing dodecyl-substituted tetrabenzopentacene product 4 in a more modest yield of 26% (see Experimental I, II, and ESI Fig. S5 and S6 †). 32,42These experiments demonstrated that our tetrabenzopentacene was readily modied with extended alkyl chains, which can mitigate aggregation and enhance solubility for acenes. 1,2,39e continued our efforts by synthesizing a nitrogencontaining tetrabenzoheptacene derivative (Scheme 2).5][36][37][38][39][40][41] Next, we cyclodehydrohalogenated diquinolineanthracene variant 6 under our validated standard conditions, forming two intramolecular carbon-carbon bonds and furnishing dodecyl-substituted tetrabenzoheptacene product 7 in a comparable yield of 32% (see Experimental I, II, and ESI Fig. S13 and S14 †). 32,42These experiments demonstrated that our tetrabenzopentacene's core aromatic motif was readily expanded to a heptacene, which can possess improved functional properties and serve as a model longer acene. 3,17,43e extended our efforts by synthesizing nitrogen-containing tetrabenzopentacene derivatives featuring amino and bromo functional groups (Scheme 3).5][36][37][38][39][40][41] Next, we reduced 9's nitro groups using routine catalytic transfer hydrogenation conditions (i.e., chloroform/ethanol/water as the solvents and Fe/CaCl 2 as the reducing reagents), forming two 6aminoquinoline moieties and generating intermediate 10 in a good yield of 62% (see Experimental I, II, and ESI Fig. S21, and S22 †). 44Then, we cyclodehydrohalogenated diquinolineanthracene variant 10 under our validated standard conditions, forming two intramolecular carbon-carbon bonds and furnishing amino-functionalized tetrabenzopentacene product 11 in a moderate yield of 20% (see Experimental I, II, and ESI Fig. S23 and S24 †). 32,42Last, we converted tetrabenzopentacene 11's amino groups to bromo groups using established Sandmeyer-type reaction conditions (i.e., acetonitrile/bromoform as the solvents and NaNO 2 /KBr/PTSA as the acid system), affording bromo-functionalized tetrabenzopentacene product 12 in a moderate yield of 17% (see Experimental I, II, and ESI Fig. S25 and S26 †). 45][48][49] We further advanced our efforts by synthesizing a dimeric nitrogen-containing tetrabenzopentacene derivative (Scheme 4).First, we reacted 1,5-dichloro-9,10-diethynylanthracene 1 with 3,5-di-tert-butylphenyl-substituted 4-nitrophenylaldimine 8 under slightly modied milder aza-Diels-Alder reaction conditions (i.e., at a lower temperature), forming a nitroquinoline moiety and generating intermediate 13 in a good yield of 45% (see Experimental I, II, and ESI Fig. S27 and S28 †).S35 and S36 †). 45Last, we cross-coupled the bromo groups of two tetrabenzopentacene 18's with each other using well-known Ullmann reaction conditions (i.e., dimethylformamide/toluene as the solvents and Ni(COD) 2 /COD/bpy as the transition metal catalyst system), affording dimeric tetrabenzopentacene product 19 in a reasonable yield of 39% (see Experimental I, II, and ESI Fig. S37 and S38 †). 50These experiments demonstrated that our tetrabenzopentacene could be site-specically substituted with single halogen functional groups, which are amenable to many routine cross-coupling reactions, [46][47][48][49] and moreover could be readily assembled into larger molecular frameworks, which opens opportunities for the synthesis of corresponding nitrogen-containing graphene nanoribbons. 16,34,51e last investigated the electronic properties of our tetrabenzoacene derivatives by means of UV-Vis spectroscopy (Fig. 2).First, the spectrum obtained for dodecyl-substituted tetrabenzopentacene 4 featured prominent absorption peaks with maxima at 641 nm and 592 nm as well as a shoulder at 541 nm, with the addition of extended side chains mitigating aggregation but not signicantly shiing the absorption peaks relative to those of the analogous previously-reported tert-butyl-substituted tetrabenzopentacene (Fig. 2 and ESI Fig. S39 †). 32econd, the spectrum obtained for dodecyl-substituted tetrabenzoheptacene 7 featured prominent absorption peaks with maxima at 648 nm and 598 nm as well as a shoulder at 547 nm, with the extension of the aromatic core only slightly red shiing the absorption peaks relative to those of the analogous tertbutyl-and dodecyl-substituted tetrabenzopentacenes (Fig. 2 and ESI Fig. S39 †).Third, the spectrum obtained for aminofunctionalized tetrabenzopentacene 11 featured prominent absorption peaks at 668 nm and 622 nm as well as a small shoulder at 554 nm, with the introduction of the two electrondonating amino groups substantially red-shiing the absorption peaks relative to those of the analogous tert-butyl-and dodecyl-substituted tetrabenzopentacenes (Fig. 2 and ESI Fig. S39 †).Fourth, the spectrum obtained for bromo-functionalized tetrabenzopentacene 12 featured prominent absorption peaks at 623 nm and 576 nm as well as a shoulder at 521 nm, with the introduction of the two electron-withdrawing bromo groups substantially blue-shiing the absorption peaks relative to those of the analogous tert-butyl-and dodecyl-substituted tetrabenzopentacenes (Fig. 2 and ESI Fig. S39 †).Last, the spectrum obtained for dimeric tetrabenzopentacene 19 featured prominent absorption peaks at 631 nm and 590 nm and no obvious shoulders, with the covalent linkage of the tetrabenzopentacene moieties causing aggregation and broadening/blue-shiing the absorption peaks relative to those of the monomeric tetrabenzopentacenes in agreement with precedent for some previously-reported acenes (Fig. 2 and ESI Fig. S39 †). 52,53Overall, these measurements showed that the electronic properties of our nitrogen-containing tetrabenzoacene derivatives could be controlled to some extent via the described synthetic strategies.

Conclusion
In summary, we have described the synthesis and characterization of substituted, expanded, functionalized, and dimeric nitrogen-containing tetrabenzoacenes, and our ndings hold signicance for several reasons.First, the modication of our tetrabenzopentacenes with extended alkyl chains not only enhances solubility and mitigates aggregation but could also eventually simplify their processing from common organic solvents within the context of transistor applications. 1,2,7,9,10,19econd, the straightforward substitution of our tetrabenzopentacenes with electron-donating amino and electronwithdrawing bromo groups readily affords tuning of their optical and electrochemical properties in a manner advantageous for optoelectronics applications. 1,2,11,16,18,197][48] Last, the expansion and dimerization of our tetrabenzopentacenes suggests that they may prove useful as model compounds for nitrogen-containing acenes and could even serve as precursors for nitrogen-doped graphene nanoribbons. 3,16,17,34,43,51Overall, our synthetic methodology substantively adds to the small number of tetrabenzoacene derivatives reported to date and may open further opportunities for these materials in organic optoelectronics applications.

Experimental I. General methods
A. Materials and conditions.All chemicals and solvents were purchased from Sigma Aldrich, Acros Organics, Combi-Blocks, or Thermo Fisher Scientic.Toluene and chloroform were routinely dried with 3 Å molecular sieves and stored under argon.The glassware was oven dried at temperatures of 150-200 °C.The reactions were performed under dry argon unless otherwise noted.Additional protocols are noted below for specic compounds where appropriate.
B. Compound purication.Flash chromatography was performed using a CombiFlash Rf 200 purication system (Teledyne ISCO, Inc.) according to the manufacturer's recommended protocols.Chromatography solvents are reported as percentages followed by solvent combinations.Additional purication-relevant information is noted below for specic compounds where appropriate.
C. Nuclear magnetic resonance (NMR) spectroscopy characterization.The intermediates and products were characterized with 1 H and 13 C nuclear magnetic resonance (NMR) spectroscopy in the University of California, Irvine Nuclear Magnetic Resonance Facility.The NMR measurements were performed on either a Bruker DRX500 instrument outtted with a CryoProbe (Bruker TCI 500 MHz, 5 mm diameter tubes) or an AVANCE600 instrument outtted with a CryoProbe (Bruker CBBFO 600 MHz, 5 mm diameter tubes).The NMR experiments were typically performed at compound concentrations of ∼1 mg mL −1 to ∼10 mg mL −1 .The chemical shis were reported in ppm for both the 1 H and 13 C NMR spectra.The chemical shis for the NMR data were referenced as follows: for compounds in CDCl 3 , the 1 H NMR spectra were referenced to tetramethylsilane (TMS) at 0.00 ppm or the residual CHCl 3 peak at 7.26 ppm, and the 13 C NMR spectra were referenced to the residual CHCl 3 peak at 77.16 ppm; for compounds in CS 2 , the 1 H NMR spectra were referenced to TMS at 0.00 ppm, and the 13 C NMR spectra were referenced to TMS at 0.00 ppm.Both the 1 H and 13 C NMR data were labeled with the chemical shis, multiplicities (s = singlet, d = doublet, t = triplet, q = quartet, quint = quintet, m = multiplet, br s = broad singlet), coupling constants in Hertz, and integration values.The NMR spectra were processed and analyzed by using the MestReNova soware package.
Fig. 2 The UV-Vis absorption spectra obtained for 4 (black trace), 7 (red trace), 11 (blue trace), 12 (green trace), and 19 (purple trace).Note that the absorption spectra were normalized by using their maximum absorbance peak within the 600 nm to 700 nm wavelength range in order to facilitate direct comparisons.
D. Mass (MS) spectroscopy characterization.The intermediates and products were characterized with electrospray ionization (ESI) high resolution mass spectrometry (HRMS) or matrix-assisted laser desorption/ionization-time of ight (MALDI-TOF) mass spectrometry at the University of California, Irvine Mass Spectrometry Facility.The HRMS measurements were performed on a Waters LCT Premier time-of-ight instrument, and the MALDI-TOF measurements were performed on an AB SCIEX TOF/TOF™ 5800 series mass spectrometer using a 349 nm Nd:YAG laser with either TCNQ or dithranol as the matrix.The mass spectra were processed and analyzed by using the standard MassLynx and TOF/TOF Series Explorer soware packages.
E. Ultraviolet-visible (UV-Vis) spectroscopy characterization.The products were characterized with UV-Vis spectroscopy at the University of California, Irvine Laser Spectroscopy Laboratory.The measurements were performed on a Jasco-V670 Absorption Spectrometer.The measurements were conducted in ambient atmosphere at room temperature.The UV-Vis spectroscopy experiments were repeated for at least three independently prepared solutions in CHCl 3 for each compound at concentrations between ∼10 mM and ∼41 mM (Fig. 2) or at concentrations between ∼35 mM and ∼84 mM (Fig. S39 †).The spectra were processed and analyzed with the Jasco Spectra Manager Suite and Origin soware packages.